To increase the memory capacity of the non-volatile semiconductor memory unit such as an EEPROM (Electrically Erasable and Programmable Read Only Memory) and an electrically collectively erasable flash memory, a so-called multi-value level type has been proposed in which tertiary or more data is stored in a single memory cell (1992, Autumnal 53rd Convention Report of the Japan Society of Applied Physics, page 653)
The above report, however, proposes only a method of storing data in the multi-value level type non-volatile semiconductor memory unit and does not propose a method of rewriting stored data.